In industrial installations using screwing elements, during the tightening and slackening operations, a pull is exerted on the said elements by means of a tension device, in order to keep them in a prestressed state.
This applies particularly to pressure vessels used, for example, in the nuclear or petrochemical industries in particular, where it is necessary to screw or unscrew several pins and also, by means of a single device, to exert a pull on all these pins, for example, the pins for fastening a flange or a plug to a passage hole for servicing work.
For this purpose, FR-A-No. 2,329,414 discloses a tension device composed of a set of hydraulic tensioners fastened and distributed in polar symmetry on a carrier ring. The hydraulic tensioners act simultaneously and achieve the desired prestress on each pin. To obtain the desired force, each hydraulic tensioner has several superposed stages, each comprising a piston equipped with a central bore, through which the pin shank passes.
However, because of the superposed active chambers, in this device the axial overall size is large. and production and assembly are complicated by the number of components to be machined, by the elements for the distribution of working fluid and also by the considerable number of sealing means.
FR-A-No. 2,274,997 discloses a device for screwing and unscrewing under tension in order to open or close the cover of a nuclear reactor vessel, comprising a carrier ring equipped with lifting jacks which, under pressure, subject the vessel pins to tension. This device uses a pair of single-stage jacks for each pin, thus considerably increasing the number of jacks on the carrier ring. Furthermore, the double thrust of the two jacks on each screwed element can cause force losses in relation to the axial direction, either as a result of tilting or as a result of the deformation of the screwed element.
Finally, FR-A-No. 2,564,767 discloses an apparatus for screwing and unscrewing a pin under tension, comprising a multiple-jack assembly, each of the jacks of which is associated with a screwed element. However, in this device, the symmetrical arrangement is in one and the same plane of the jacks, thus limiting the cross-section of the working chambers if it is necessary to have a higher force threshold, while maintaining an equivalent working-fluid pressure.